1. Field of the Invention
The invention relates to a process for the localization of multiple-scintillation events in a gamma camera having a number of photomultipliers whose output signals, in dependence on a release signal, are subjected to a pattern recognition process.
2. Description of the Prior Art
One of the limiting factors for the performance capacity of a gamma or Anger camera is the achievable counting rate. The limit with current systems lies typically at approximately 200,000 events per second. The main reason for this is that the gamma cameras in their present state still only analyze single event within 1 to 2 .mu.s, and identify it as usable for the nuclear medical image. The selection of valid events occurs, for example, over an energy window, i.e., over the impulse height at the output of a detector.
If two or more gamma quanta arrive simultaneously or virtually simultaneously in the detector, their total energy can exceed the window for single events. The quanta, however, are rejected in conventional cameras, so that the counting rate is clearly reduced. If the time spacing between the two individual events is sufficiently large, special switching circuits there can be used to ascertain a so-called pile-up or a double event. At best, then, the first event can be approximately analyzed and used. The second is rejected, so that again there is a considerable loss in the counting rate. If the time spacing of a double event is too small, both events must be rejected.
In European Application 0,155,463 an analog circuit arrangement is disclosed for the analysis of multiple events. For this purpose a network is provided for communication between the photomultipliers of the gamma camera, with which multiple events can be detected and separated, but only if their spacing is so large that spatially separate signal groups can be formed. Even with a large spacing of the double events the signal groups can mutually interfere, so that the analysis and localization of the two constituent single events are falsified.
A process of the type initially mentioned at the outset is described in the article by Milster et al. "A Full Field Modular Gamma Camera", published in The Journal of Nuclear Medicine, vol. 31, No. 4, April 1990, pages 632 to 639. A modular gamma camera is described therein which delivers useful image information data over the entire crystal surface. The localization occurs after digitalization of the output signals of the photomultipliers with the aid of the maximum-likelihood function. Each module is optically and electronically independent, so that several modules can be combined into a camera system with increased counting rate. This articles does not mention, however, for the increase of a counting rate, also localizing multiple events.